In this connection, "materials similar to wood which are easily machined" is understood to mean, for example, other wood materials, plastic materials or materials agglomerated therefrom. Such a milling tool is in particular intended to be used for processing, for example trimming plate-shaped components made of materials like that.
Manufacturers of woodworking machines have been trying for decades to reduce the level of sound output radiated by the machines, or respectively to keep it within bearable approved limits. Although the mechanism of sound generation can be considered to be scientifically understood (Troger, J.: "Uber den Mechanismus der Schallentstehung beim Spanen" [On the Mechanism of Sound Generation During Machining]--1st Report: "Untersuchung uber den Arbeitslarm" [Studies Regarding Workplace Noise], Holztechnology [Wood Technology] Leipzig 10 (1969) 3, pp. 181 to 184;--2nd Report: "Untersuchungen uber den Leerlauflarm" [Studies of Noise When Running Idle], Wood Technology 10 (1969) 4, pp. 265 to 269;--3rd Report: "Theoretische Untersuchungen" [Theoretical Studies] Part I, Wood Technology Leipzig 11 (1970) 1, pp. 41 to 47; 4th Report: "Theoretische Untersuchungen" [Theoretical Studies] Part II, Wood Technology Leipzig 11 (1970) 2, pp. 75 to 80), there has been no success in completely meeting the prescribed legal requirements in connection with trimming work. The introduction of diamonds as the cutting material in circular sawing tools and compact cutting tools, which has occurred in recent years, required increased wedge angles, because of which the specific cutting force increased approximately tenfold. With this, the emitted sound output level also increased, since there are significant connections between the cutting force and noise emission. Machines for trimming to shape in the furniture industry are particularly affected, of which in Germany alone there are approximately 500 machines combined into installations. For this reason there exists a need for action with the goal of clearly reducing the presently too high noise levels of approximately 90 to 95 dB AI and more at the respective work stations. New basic solutions for reducing the emitted sound output must be sought, based on the known mechanisms of sound generation. Besides a reduced noise emission, the tools with these special cutting geometry also must assure excellent processing qualities. As the latest publications in the trade journals show, the demands for high-grade processing quality of the products of the wood processing and wood-working industry are still quite topical.
Sound sources in the course of processing wood are the tool and the workpiece. Both the noise while idling as well as the noise being emitted when machining (work noise) can be determining. The main sound source during idling is the tool. The aerodynamic eddy noise is created by air vortices being alternatingly released at the sides of the cutting edges. This noise is characterized by a broad band roar without distinguished tonal components. If there is a sharp-edged obstacle in the vicinity of the vortex field, the so-called "sound of rotation" appears, which has a distinguished tonal component, whose frequency equals the product of the number of cutting edges and the rpm. The increase in the sound output level with the reduction of the distance from the cutting edge can be up to 10 dB. In connection with the interpretation of the work noise it must be assumed that the tool and the workpiece are structures which are capable of mechanical oscillations. Their excitation during the milling process is caused by a portion of the milling force. It is not possible to precalculate the size of the excitation force, in accordance with the above mentioned scientific studies it is proportional to the cutting force. In the same way it is not possible to exactly define the workpiece as a mechanical oscillator.
The cutting force consists of a frictional portion, which is constant during the operation, and a portion which is proportional to the instantaneous depth of cut. Since the depth of cut increases (opposed running) or decreases (ganging) linearly over time, this portion is triangular, the frictional force portion is constant (rectangular) during the operation. An oscillation excitation of the tool and the workpiece takes place both at the front of the cutting operation as well as at the end of the contact of the cutting edge with the workpiece, i.e. respectively at the time at which the greatest discontinuity of the excitation force is present.
In actuality, the problems in connection with trimming are caused less by the idling noise than by the work noise. The radiated acoustic output (P.sub.ak) is proportional to the sound-radiating surface (A), the square of the effective value of the oscillation velocity (v.sub.eff), the air density (p), the speed of sound (c) and the degree of radiation (.sigma.): EQU P.sub.ak =.sigma..multidot.p.multidot.c.multidot.v.sub.eff.sup.2 .multidot.A (1)
The higher the effective value of the oscillation velocity, the greater the emitted sound output level, and vice versa. Even if it would be possible to exactly calculate the effective value of the oscillation velocity, there are considerable problems in connection with the determination of the degree of radiation (.sigma.) and the radiating surface (A). A precalculation of the radiated acoustic output (P.sub.ak) of a particular system capable of oscillating is extremely difficult, if possible at all. In spite of these problems it was possible to qualitatively prove the effects of the technical milling actuating variables of the milling operation on the sound emission. By means of a model-like reproduction of the milling and oscillating process it was possible to prove a good agreement with reality. The simulation of the oscillating processes during milling took place in the above mentioned scientific studies by means of a passive analog computer, in which all parameters of interest, such as the type and number of the supports, damping, density, modulus of elasticity, spring constants and forces (frequency, length of time, shape of the pulse, pulse duty factor) were simulated. All technical milling actuating variables which, in the simulation of the cutting force resulted in an increase of the effective value of the oscillation velocity of the model, also caused an increase of the sound level. In this way it was possible, for example, to prove the effects of the rate of chip removal both in connection with the velocity level of the model as well as with the actual sound output level with a log. graduation of the abscissa to be a linear connection. Here the oscillation velocity during simulation of the opposed running is considerably less at the start of the cutting force impulse (small rise of dF/dt) than at the sudden exit of the cutting edge from the workpiece (large rise of dF/dt). In 1971 the claim was formulated in the above mentioned scientific studies that by reducing the unsteadiness of the chronological progression of the milling force it would be possible to reduce the oscillation velocity and therefore the noise emission. Niemeyer, W.-H. in: "Primare Linderungsmasnahme in der Holzbearbeitung: Fraswerkzeuge" [Primary Reduction Step in Woodworking: Milling Tools], Industrie-Anzeiger [Industrial Gazette] 110 (1988), No. 23, pp. 40 and 41, confirmed this realization in 1988 by his studies. The size of the exciting force is not decisive, but its unsteadiness is. The exciting force of the system of tool/workpiece capable of oscillations is proportional to the portion of the milling force which acts in the direction, or respectively directions, in which the particular system can oscillate. The calculation of the excitation force for a particular case is difficult, since only a few studies regarding the size and direction of the milling force are available. Additional scientific realizations are available from Heisel, U., Troger, J., Dietz, H.: "Am Schneidkeil wirkende Krafte" [Forces Acting on the Cutting Wedge], Part (1), Holz- und Kunststoffbearbeitung [Wood and Plastic Material Processing], HK (1995) 6, pp. 884 to 888; Part (3) Holz- und Kunststoffbearbeitung [Wood and Plastic Material Processing], HK (1995) 7, pp. 1000 to 1004. For simulating the system, the cutting force was selected as the excitation force and it was possible to interpret the qualitative connections quite well by means of this, so that the behavior of the excitation force can be explained with the aid of the cutting force. A force resulting from the milling process only acts if a cutting tooth is in engagement.
A tooth of a circular saw which, for example, operates in the opposed direction, does not encounter the workpiece with its cutting edge, but suddenly with its breast (cutting surface). It is understood that at the moment of the impact of the cutting surface of the cutting wedge there is no ideal cutting process. The increase of cutting force of an opposite running circular saw takes place over a length of time during which the tool (for example for a tooth advance F.sub.z =0.5 mm) turns around an angle of rotation of approximately .phi.=0.2.degree.. This corresponds to a time of approximately 5.times.10.sup.6 s. In ganging, which is mainly used in trimming, the cutting edge impacts directly on the surface of the workpiece. In this case the increase is even greater, the cutting force here acts immediately for all practical purposes. It is necessary to drastically reduce the increase of the cutting force-time function for achieving a noise reduction.